Polyurethanes which are formed by the reaction of the hydroxyl groups of a polyol with a curing agent, such as an organic isocyanate compound, provide excellent bonding or joining properties when cured, thus forming strong adhesives. By utilizing an isocyanate terminated prepolymer technique, it is possible to obtain moisture curable formulations which provide similar adhesive properties.
These compounds are not useful in certain applications, however, because they exhibit very little green strength, if any, before fully curing. "Green strength" as used herein means the ability of the adhesive to hold two surfaces together when first contacted and before the adhesive develops its ultimate bonding properties when fully cured.
The degree of green strength exhibited by an adhesive is very important in many applications. High green strength adhesives tend to prevent wrinkling and slippage of films during lamination. In panel assembly and packaging, faster handling and wrapping rates can be achieved with such high green strength adhesives. When adhesives are applied to a vertical surface, a sufficiently high green strength allows the adhesive to prevent a mechanically unsupported, bonded member from slipping under the influence of gravity. When employed for flocking, a high green strength adhesive holds the fibers in place while curing. Also, in the laying of carpet or synthetic flooring, adhesives having a high green strength resist curling due to the shape memory of the flooring which is acquired when stored in a roll.
Attempts to increase the green strength of isocyanate terminated prepolymers by adding other high polymers such as polyvinyl acetate, cellulose acetate butyrate, acrylics, polyketones, phenoxy and polyvinylchloride-vinyl acetate copolymers have proven unsuccessful. Other formulations suggest the use of chlorinated rubber or chlorinated polyvinyl chloride in conjunction with ricinoleate urethane polyols or prepolymers. These are described in U.S. Pat. Nos. 4,217,254, 4,256,615, 4,340,682, and 4,569,572. The present invention discloses new and unique compositions which provide an additional solution to this problem.